This invention lies in the area of leads for delivering electrical signals to body organs and, more particularly, epicardial leads for use in cardiac pacing systems and having means for fixation to the heart wall.
In cardiac pacing systems it is known that in a certain proportion of patients it is desirable to utilize an epicardial lead, to ensure good fixation and permanent engagement of the lead to the heart wall. This is the case primarily because in a certain number of patients proper lead fixation cannot be achieved with endocardial leads, or catheters, and in order to obtain fixation with a desirable threshold an epicardial type lead is required. A number of epicardial leads are presently available for use with pacing systems, the most common forms involving a screw type element at the distal end of the lead, which element serves both as the stimulus electrode and as the fixation member. In these prior art arrangements, the fixation member is integrally connected to the lead, and while it may be flexible so as to facilitate engagement with the heart wall, the fixation member generally is not movable relative to the lead itself. This prior art feature places certain constraints upon lead design, as well as upon the design of insertion tools used for actually fixing the epicardial lead to the patient's heart. Examples of prior art epicardial leads and insertion assemblies include those disclosed in U.S. Pat. Nos. 4,007,745, 3,737,579 and 3,875,947.
There are a number of characteristics of the heretofore available epicardial leads and systems for inserting same which are considered to be undesirable and which need improvement. For example, the fixation means, which generally is a screw or "corkscrew" type element is fixed to the lead in such a manner that rotation of the corkscrew element into the heart wall involves rotation of the remainder of the lead itself, necessitating special insertion tools for avoiding placing torque on the remainder of the lead. Additionally, there is no restraint upon the ability of the operator to continue to turn the corkscrew element after it has been fully threaded into the heart wall. It is well known that if the corkscrew element is turned after it has been fully extended into the heart wall there is resulting trauma to the myocardial tissue, and fibrosis. Further, it is unpractical to use a screw type element to probe for a proper position which gives a desirable threshold, since repeated engagement and disengagement of such a corkscrew type element causes unwanted if not intolerable trauma to the myocardial tissue. If a separate probe element is used to determine a position for obtaining a good threshold placement, this is still a very unsatisfactory technique since the probe element must be removed and the corkscrew element inserted. Generally the results are not optimal, both because it is difficult to reinsert the corkscrew element at the same place as the probe had been inserted, and because the surface characteristics (and thus the threshold characteristics) of the corkscrew element are different from the probe which had been used.
In other prior art leads, an attempt is made to screw into the heart tissue axially, by somehow applying rotational force from the proximal end of the lead. See, for example, U.S. Pat. Nos. 3,827,428 and 3,974,834.
There is thus a substantial need in cardiac pacing systems for a means and a method for efficient and reliable fixation of an epicardial lead, providing for optimum contact electrode placement and maximum secure fixation.